1. Field of the Invention
The present invention is related to a protect-control device capable of limiting current for reducing noise resulting from switchover of motor and particularly to a device capable of restricting noise of a running motor by means of limiting driving voltage level of the motor.
2. Brief Description of the Related Art
The conventional art usually employs pulse width modulation signal (PWM) to control conducting time of the coil set in a cooling fan motor so as to control rotation of the cooling fan motor. Further, different rotational speeds can be controlled in accordance with different duty cycles of the PWM signal. However, amplitude, frequency and duty cycle of the PWM are variables to affect the rotational speed of the fan motor.
Besides, referring to FIG. 1, the first conventional circuit is illustrated. The first conventional circuit includes a reverse voltage protective diode 101, a Hall component 102, a driving component 103, a first transistor 104, a second transistor 105, a first resistance 106, a second resistance 107, a third resistance 108 with a first motor coil set 109, a second motor coil set 120, a fourth resistance 121, a fifth resistance 122, a third transistor 123 and a fourth transistor 124. The Hall component 102, the driving component 103, the first transistor 104, the second transistor 105, the first resistance 106, the second resistance 107, the third resistance 108, the first motor coil set 109 and the second motor coil set 120 constitute a driving circuit 10.
When the power is conducted to the circuit via the reverse voltage protective diode 101, the Hall component 102 of the driving circuit 10 detects the magnetic pole change of the rotor to produce positive voltage H+ and negative voltage H− outputting to the driving component 103. The driving component 103 transforms the preceding voltages as control signal source being output to the first transistor 104 and the second transistor 105 via the first output end OUT1 and the second output end OUT2 respectively. A PWM signal from PWM input end passes through the fourth resistance 121, the fifth resistance 122 and both collectors of the third transistor 123 and fourth transistor 124 to cut off control signal sources of the first output end OUT1 and the second output end OUT2 such that the first motor coil set 109 and the second motor coil set 120, which are connected to collectors of the first transistor 104 and the second transistor 105 respectively, are controlled for changing the rotational speed of the fan motor.
Referring to FIG. 2, the second conventional circuit is illustrated. The second conventional circuit includes a reverse voltage protective diode 201, a Hall component 202, a driving component 203, a first transistor 204, a second transistor 205, a first resistance 206, a second resistance 207, a third resistance 208, a first motor coil set 209, a second motor coil set 220, a fourth resistance 221 and a third transistor 222. The Hall component 202, the driving component 203, the first transistor 204, the second transistor 205, the first resistance 206, the second resistance 207, the third resistance 208, the first motor coil set 209 and the second motor coil set 220 constitute a driving circuit 20′. A PWM signal from a PWM input end is connected to a control input end ST of the driving component 203 via the fourth resistance 221 and the third transistor 222 such that the driving circuit 20′ is controlled to produce switch actuating time by means of the duty cycle of the PWM signal being changed in accordance with periodic change of PWM signal. As a result, power of the first motor coil set 218 and the second motor coil set 219 can be controlled for controlling the rotational speed of the fan motor.
Referring to FIG. 3, the third conventional circuit is illustrated. The driving circuit 30′ is composed of a reverse voltage protective diode 301, a Hall component 302, a driving component 303, a first transistor 304, a second transistor 305, a first resistance 306, a second resistance 307, a third resistance 308, a first motor coil set 309 and a second motor coil set 320. The PWM signal from a PWM input end passes through the fourth resistance 321 and the third transistor 322 to control “On” and “Off” of the power such that the first motor coil set 309 and the second motor coil set 320, which are connected to collectors of the first transistor 304 and the second transistor 305 respectively, are controlled for changing the rotational speed of the fan motor.
Referring to FIG. 4, wave curves of the PWM signal with respect to time and currents passing through the motor coil sets 309, 320 shown in FIG. 3 with respect to time are illustrated. It can be seen in the figure that once the prior art needs to limit the current level passing through the motor coil sets for meeting different applications, 25 KHZ PWM signal is off till the motor coil current being lower than a preset level due to the driving component performing function of limiting current of protection. In this way, a plurality of 25 KHZ PWM signals becomes disappeared and frequencies of the PWM signals become lower than 20 KHZ such that it results in generation of the noise while the rotational speed of the motor is switched over.